TAXONOMIC STUDIES OF RICKETTSIELLA, RICKETTSIA, AND CHLAMYDIA USING GENOMIC DNA

Invertebrate pathogens of the genus Rickettgiella (Order Rickettsiales ) undergo a developmental cycle more characteristic of chlamydial orga nisms (Order Chlamydiales) than of typical rickettsiae. Moreover, amon g recognized species of Rickettsiella, there is considerable variation in host range and in the ultrastructure and development of the infect ious stage, i.e., the elementary body. To begin an analysis of the tax onomic relationships of the invertebrate pathogens belonging to the ge nus Rickettsiella and of the relationship of these to chlamydiae and o ther rickettsiae, genomic DNA of representative species was compared w ith respect to restriction enzyme site polymorphism, DNA-DNA hybridiza tion, G + C ratios, and fine melting profiles. The strains studied inc luded isolates currently classified as Rickettsiella grylli, R. popill iae, R. chironomi, Chlamydia psittaci, C. trachomatis, Coxiella burnet ii, and Rickettsia conorii. Reciprocal DNA hybridization studies carri ed out under high-stringency conditions (65-degrees-C) showed homology between R. grylli and R. popilliae isolates but not with the others, including two isolates of R. chironomi from the midge, Chironomus dors alis, and the scorpion, Buthus occitanus. The R. chironomi isolates sh ared no detectable homology with each other or with any of the other s trains or species. Additionally, no homology was detected between any of the Rickettsiella isolates and the species of Coxiella, Rickettsia, or Chlamydia. Results obtained from the DNA fine melting profiles and G + C ratios corresponded with the results obtained from the DNA hybr idization studies. Thus, the present study indicates that, despite sim ilarities in developmental cycles, Rickettsiella and Chlamydia are tax onomically distinct groups appropriately placed in different orders. H owever, the uniqueness of the two R. chironomi species, both from each other and from the other organisms studied, indicates these could be recognized as distinct species. (C) 1994 Academic Press, Inc.